Lecture 4

OUTLINE• Bipolar Junction Transistor (BJT)

– General considerations

– Structure

– Operation in active mode

EE105 Fall 2007 Lecture 4, Slide 1 Prof. Liu, UC Berkeley

– Operation in active mode

– Large-signal model and I-V characteristics

Reading: Chapter 4.1-4.4.2

Voltage-Dependent Current Source

• A voltage-dependent current source can act as an

amplifier.

• If KRL is greater than 1, then the signal is amplified.

EE105 Fall 2007 Lecture 4, Slide 2 Prof. Liu, UC Berkeley

L

in

outV KR

V

VA −=≡

Voltage-Dependent Current Source

with Input Resistance

• The magnitude of amplification is independent of the input resistance rin.

EE105 Fall 2007 Lecture 4, Slide 3 Prof. Liu, UC Berkeley

Exponential Voltage-Dependent Current Source

• Ideally, a bipolar junction transistor (BJT) can be modeled as a three-terminal exponential voltage-dependent current source:

EE105 Fall 2007 Lecture 4, Slide 4 Prof. Liu, UC Berkeley

Reverse-Biased PN Junction

as a Current Source

• PN junction diode current is ~independent of the

reverse-bias voltage. It depends only on the rate at

which minority carriers are introduced into the

depletion region.

⇒ We can increase the reverse current by injecting

EE105 Fall 2007 Lecture 4, Slide 5 Prof. Liu, UC Berkeley

⇒ We can increase the reverse current by injecting

minority carriers near to the depletion region.

BJT Structure and Circuit Symbol

• A bipolar junction transistor consists of 2 PN junctions that form a sandwich of three doped semiconductor regions. The outer two regions are doped the same type; the middle region is doped the opposite type.

EE105 Fall 2007 Lecture 4, Slide 6 Prof. Liu, UC Berkeley

NPN BJT Operation (Qualitative)

In the forward active mode of operation:

• The collector junction is reverse biased.

• The emitter junction is forward biased.

EE105 Fall 2007 Lecture 4, Slide 7 Prof. Liu, UC Berkeley

B

C

I

I≡βcurrent gain:

Base Current

• The base current consists of two components:

1) Injection of holes into the emitter, and

2) Recombination of holes with electrons injected from

the emitter.

EE105 Fall 2007 Lecture 4, Slide 8 Prof. Liu, UC Berkeley

BCII β=

BJT Design

• Important features of a well-designed BJT (large β ):

– Injected minority carriers do not recombine in the

quasi-neutral base region.

�

EE105 Fall 2007 Lecture 4, Slide 9 Prof. Liu, UC Berkeley

– Emitter current is comprised almost entirely of

carriers injected into the base (rather than carriers

injected into the emitter).

�

Carrier Transport in the Base Region

• Since the width of the quasi-neutral base region (WB = x2-x1) is much smaller than the minority-carrier diffusion length, very few of the carriers injected (from the emitter) into the base recombine before they

EE105 Fall 2007 Lecture 4, Slide 10 Prof. Liu, UC Berkeley

into the base recombine before they reach the collector-junction depletion region.

� Minority-carrier diffusion current is ~constant in the quasi-neutral base

• The minority-carrier concentration at the edges of the collector-junction depletion region are ~0.

Diffusion Example Redux

• Non-linear concentration profile

� varying diffusion current

• Linear concentration profile

� constant diffusion current

dL

xNp

−= exp

−=

L

xNp 1

EE105 Fall 2007 Lecture 4, Slide 11 Prof. Liu, UC Berkeley

L

NqD

dx

dpqDJ

p

pdiffp

=

−=

,

dd

p

pdiffp

L

x

L

NqD

dx

dpqDJ

−=

−=

exp

,

Collector Current

inES

BESC

T

BE

BB

inEC

WN

nqDAI

V

VII

V

V

WN

nqDAI

2

2

whereexp

1exp

=≅

−=

EE105 Fall 2007 Lecture 4, Slide 12 Prof. Liu, UC Berkeley

• The equation above shows that the BJT is indeed a voltage-dependent current source; thus it can be used as an amplifier.

BB

S

T

SCWN

IV

II whereexp =≅

Emitter Current

• Applying Kirchhoff’s Current Law to the BJT, we can

easily find the emitter current.

+=+=1

1CBCE IIII

EE105 Fall 2007 Lecture 4, Slide 13 Prof. Liu, UC Berkeley

+=+=β

1CBCE IIII

Summary of BJT Currents

exp1

exp

=

=

βBE

SB

T

BESC

V

VII

V

VII

EE105 Fall 2007 Lecture 4, Slide 14 Prof. Liu, UC Berkeley

1

exp1

+≡

+=

β

βα

β

β

β

T

BESE

T

SB

V

VII

V

Parallel Combination of Transistors

• When two transistors are connected in parallel and have the same terminal voltages, they can be considered as a single transistor with twice the emitter area.

EE105 Fall 2007 Lecture 4, Slide 15 Prof. Liu, UC Berkeley

Simple BJT Amplifier Configuration

• Although the BJT converts an input voltage signal to an output current signal, an (amplified) output voltage signal can be obtained by connecting a “load” resistor (with resistance RL) at the output and allowing the controlled current to pass through it.

EE105 Fall 2007 Lecture 4, Slide 16 Prof. Liu, UC Berkeley

BJT as a Constant Current Source

• Ideally, the collector current does not depend on the collector-to-emitter voltage. This property allows the BJT to behave as a constant current source when its base-to-emitter voltage is fixed.

EE105 Fall 2007 Lecture 4, Slide 17 Prof. Liu, UC Berkeley

Constraint on Load Resistance

• If RL is too large, then VX can drop to below ~0.8V so that the collector junction is forward biased. In this case, the BJT is no longer operating in the active mode, and so

� There exists a maximum tolerable load resistance.BC II β<

EE105 Fall 2007 Lecture 4, Slide 18 Prof. Liu, UC Berkeley

BJT I-V Characteristics

EE105 Fall 2007 Lecture 4, Slide 19 Prof. Liu, UC Berkeley

Example

EE105 Fall 2007 Lecture 4, Slide 20 Prof. Liu, UC Berkeley

BJT Large Signal Model

• A diode is placed between the base and emitter

terminals, and a voltage-controlled current source is

placed between the collector and emitter terminals.

EE105 Fall 2007 Lecture 4, Slide 21 Prof. Liu, UC Berkeley

BJT vs. Back-to-Back Diodes

• Figure (b) presents a wrong way of modeling the BJT.

EE105 Fall 2007 Lecture 4, Slide 22 Prof. Liu, UC Berkeley